Circular polarization immunity of the cyclotron resonance photoconductivity in two-dimensional electron systems

Two-dimensional electron systems (2DES) subjected to a perpendicular magnetic field absorb electromagnetic radiation via cyclotron resonance (CR). Here we report a qualitative deviation from this well-known behavior. Our measurements in large-sized 2DES based on GaAs and HgTe reveal that the CR-enhanced photoconductivity becomes insensitive to the radiation helicity, showing almost the same signal amplitude for both CR active and inactive polarities of B, when the temperature T is lowered to that of liquid helium or below. Strikingly, the simultaneously measured CR in the transmission demonstrates a conventional strong helicity dependence for all T. In contrast similar photoconductivity measurements in graphene show no anomalies indicating an ordinary helicity-sensitivity in the whole investigated temperature range.